Question: Solve for $x$ and $y$ using substitution. ${5x-4y = -9}$ ${x = -y-9}$
Since $x$ has already been solved for, substitute $-y-9$ for $x$ in the first equation. ${5}{(-y-9)}{- 4y = -9}$ Simplify and solve for $y$ $-5y-45 - 4y = -9$ $-9y-45 = -9$ $-9y-45{+45} = -9{+45}$ $-9y = 36$ $\dfrac{-9y}{{-9}} = \dfrac{36}{{-9}}$ ${y = -4}$ Now that you know ${y = -4}$ , plug it back into $\thinspace {x = -y-9}\thinspace$ to find $x$ ${x = -}{(-4)}{ - 9}$ $x = 4 - 9$ ${x = -5}$ You can also plug ${y = -4}$ into $\thinspace {5x-4y = -9}\thinspace$ and get the same answer for $x$ : ${5x - 4}{(-4)}{= -9}$ ${x = -5}$